As a technique for monitoring by associating a tire air pressure with a wheel position, for example, as disclosed in U.S. Pat. No. 6,435,020 B1, there is a technique for associating wheel positions identified using differences in rotation speed between each of wheels with positions of the wheels each provided with an air pressure sensor that detects a tire air pressure.
However, the technique disclosed in U.S. Pat. No. 6,435,020 B1 has a problem in that, in a case in which the vehicle runs in a four-wheel drive mode, and the like, a rotation speed difference between the front and rear wheels is small so that identifying the front and rear wheel positions is difficult.